1. Field of the Invention
The present invention relates generally to a system and method for supporting the mobility of a terminal in a wireless system employing a Mobile IP (Internet Protocol), and in particular, to a system and method for providing a handover for a Mobile IP terminal in a wireless network.
2. Description of the Related Art
Generally, wireless systems are classified into a mobile communication system to which a mobile phone or a Personal Cellular System (PCS) phone is applied, a Wireless Local Loop (WLL) system, and a Wireless Local Area Network (Wireless LAN) system. Such wireless systems have been developed as independent systems. Currently, however, the wireless systems are integrating an Internet Protocol (IP). In a method proposed to integrate systems using an IP, the systems communicate using Mobile IP. In systems where terminals communicate using Mobile IP, a target with which the terminal will communicate with over a wireless channel is referred to as an access router (AR). The Wireless LAN system is the most typical example of the system in which the access router is used.
A description will now be made of a Wireless LAN system using the Mobile IP. Basically, because a Wireless LAN system communicates with an access router using a wireless channel, the system can guarantee support for the mobility of the terminals as long as the system communicates through one access router. Such partial mobility guarantee cannot fully guarantee full mobility of terminals. That is, a terminal may move to a new access router while communicating in the coverage area of a current access router. In this case, like a mobile communication system, a Wireless LAN system must provide handover of a terminal in order to guarantee mobility. That is, the Wireless LAN system should also provide seamless communication during handover by managing the locations of terminals. In order to provide the seamless communication even during handover, the Mobile IP technology is used. The Mobile IP technology is classified into Mobile IPv4 and Mobile IPv6 according to which version is being used. In the following description, a terminal using Mobile IP will be referred to as a “mobile node (MN)” regardless of the version.
Mobile IPv4 defines a home agent (HA) and a foreign agent (FA) in order to provide for the mobility of a mobile node. In addition, Mobile IPv4 performs location management on the mobile node through the HA and the FA. The HA is located in a home network of the mobile node, and the FA is located in a target network to which the mobile node is handed over. The mobile node informs the HA of its current location each time it performs handover. Then the HA stores location information of the mobile node, and when there is data to be delivered to the mobile node, the HA encapsulates the corresponding data and delivers the encapsulated data to the FA through a tunnel. Then the FA receives the encapsulated data from the HA and delivers the received data to the mobile node located in its network.
As described above, the data delivered to a mobile node always passes through the HA, so that a data delivery route is not optimized. In order to solve this problem, a Route Optimization (RO) technique has been proposed. Meanwhile, Mobile IPv6 technology has been proposed to take best advantage of the IPv6 technology and to support a mobile node in an IPv6 network. In the IPv6 technology, because data is delivered to a mobile node basically using the Route Optimization technique, the mobile node becomes an end of a tunnel and decapsulates the encapsulated data.
A Mobile IP provides a handover method in Layer 3 (L3) in OSI 7 Layer. However, no discussion has been made on a handover method in Layer 2 (L2), communication in which should be completed before communication caused by a handover is performed in L3. In addition, a long time delay occurs during handover of a mobile node, thus making it difficult for the mobile node to perform a seamless communication. In order to solve this problem, Fast Mobile IP and Hierarchical Mobile IP have been proposed. Such techniques are superior to the simple Mobile IP in reducing a time delay occurring during handover. However, like the Mobile IP, these techniques also manage handover in L3. That is, L2 handover is not considered in these techniques. In other words, there has been proposed no appropriate method for optimizing the L2 handover and a mutual relation between L2 handover and L3 handover during L2 and L3 handovers based on the L2 handover optimization.
A description will now be made of a time delay occurring in an L2 handover process. A time delay occurring in the L3 handover process is caused by a characteristic of a wireless network as described below.
Neighbor wireless networks, although they use the same medium, use different channels in order to prevent interference. Therefore, to perform handover, a mobile node should acquire medium information and channel information used in a target wireless network to which it will be handed over. In order to acquire such information, the L2 of the mobile node should examine information on all media and channels available in the wireless network. A process of acquiring the information on all media and channels available in the wireless network is called “channel scanning.” After performing the channel scanning process, the mobile node selects the best medium and channel for handover. After determining the best medium and channel, the mobile node selects an access router (AR) corresponding to the determined medium and channel, and then performs the L2 handover process.
FIG. 1 is a diagram illustrating the access routers communicating via an IP network and a route of a mobile node. In FIG. 1, an Internet network 131 is an IPv4 or IPv6 network. The Internet network 131 is connected to first and second routers 121 and 122. The first and second routers 121 and 122 are connected to a plurality of access routers 111, 112, 113 and 114. In FIG. 1, the first router 121 is connected to first and second access routers 111 and 112, and the second router 122 is connected to third and fourth access routers 113 and 114, by way of example.
It is assumed that the first access router 111 has an IEEE 802.11a network channel, the second access router 112 has an IEEE 802.11b network channel, the third access router 113 has an IEEE 802.11a/b network channel, and the fourth access router 114 has an IEEE 802.11g network channel. A mobile node 101 is first connected to the first access router 111, and communicates over a wireless channel. The mobile node 101 is represented by reference numeral 101a when it is located in the coverage area of the first access router 111. The mobile node 101 is represented by reference numeral 101b when it is located in the coverage area of the second access router 112. The mobile node 101 is represented by reference numeral 101c when it is located in the coverage area of the third access router 113.
In this way, the mobile node changes the access router with which it communicates, as it moves. When the mobile node stays in the coverage area of the particular access router, there is no problem caused by handover. However, when the mobile node moves to a new access router, the mobile node should perform a handover, raising a problem caused by the handover.
For example, in some cases, a mobile node may change network coverage while communicating with the first access router 111 in network coverage area of the first access router 111 where the mobile node is currently is located. That is, as shown by an arrow 10, the mobile mode 101a may move to network coverage area of the second access router 112. When the access router is changed from the first access router 111 to the second access router 112 due to the change in network coverage area of the mobile node, the first and second access routers 111 and 112 may use different communication standards. For example, if the first access router 111 uses an IEEE 802.11a standard and the second access router 112 uses an IEEE 802.11b standard, the following problem can arise because they use different access techniques.
At first, because the mobile node 101a cannot know the communication standard and a channel used in a neighbor network, the mobile node 101a should detect the communication standard and a channel used in the neighbor network. In order to detect the communication standard and channel in use, the mobile node scans all channels defined in the IEEE 802.11a standard used in the network to which it is currently connected. However, in the neighbor network using the IEEE 802.11b standard, the mobile node cannot acquire the information. Then the mobile node again scans the channels using its available access standard other than the IEEE 802.11a standard. If the mobile node can use all of the IEEE 802.11a, IEEE 802.11b and IEEE 802.11g standards, it selects one of the IEEE 802.11b and IEEE 802.11g standards and again scans channels of the neighbor network through the selected communication standard. If the IEEE 802.11b standard is selected, the mobile node can detect a channel used in the neighbor network. However, if the IEEE 802.11g standard is selected, the mobile node should scan all of the channels using the IEEE 802.11g standard and then, again scan a channel of the neighbor network using the IEEE 802.11b standard.